Vehicles having internal combustion engines typically utilize intake manifold vacuum for power accessories and facilitating certain emission control activities. In particular, engines utilize intake manifold vacuum to draw stored fuel vapors from a carbon canister or other vapor storage device. In this way, fuel vapors generated in the fuel tank can be contained and then used in the engine to reduce emission of such vapors.
Various types of engine operation can affect the level of vacuum in the intake manifold, such as variation in the engine load, engine air-fuel ratio, engine valve timing and/or lift, cylinder deactivation, and engine combustion mode (such as homogenous charge compression ignition operation, HCCI), for example. Under some conditions, such engine operation can reduce available vacuum below that needed to purge sufficient fuel vapors. Thus, some approaches adjust engine operation (e.g., by adjusting air-fuel ratio, valve timing, throttling, etc.) to manage the intake manifold vacuum, while others may utilize a vacuum pump to generate additional vacuum when needed.
However, the inventors herein have recognized several issues with such approaches. While adjusting engine operation may be appropriate under some conditions, it may also result in lost fuel savings due to an inability to operating in a more efficient combustion mode. For example, due to a need to purge fuel vapors, the engine may operate in more efficient combustion modes, such as HCCI, less often than otherwise possible. Also, throttling to generate vacuum may increase engine pumping work. Further, utilizing external vacuum pumps or other such devices can also increase parasitic losses and thus degrade fuel economy, in addition to increasing cost.
The inventors herein have further recognized that it may be beneficial to push the vapors from the canister into the intake manifold using exhaust pressure, rather than, or in addition to, pulling the vapors using manifold vacuum. In this way, it may be possible to enable additional operation at lower vacuum levels, thus extending more fuel efficient combustion modes, for example.
Further, increased temperature from the exhaust gas may enable more efficient purging under some conditions. Specifically, the higher temperature of the exhaust gas (compared with fresh air) may help purge fuel vapors from a vapor storage device, such as a charcoal canister since vapor purging is an endothermic reaction. In other words, the charcoal canister normally cools when fresh air is used for purging. Using at least some exhaust gas for purging would raise the temperature and thus enable purging with a smaller volume of gas, further reducing the need for intake manifold vacuum.
Note that there are various sources of exhaust gas that may be used to purge fuel vapors, such as exhaust gas recirculation gas, or other exhaust gas.